1. Technical Field
The present invention relates to a glass fine particle synthesis burner (referred to hereinafter as the “burner”) that is used when manufacturing an optical fiber base material, a method of manufacturing a glass fine particle deposition body (referred to hereinafter as the “soot”) using the burner, and a glass fine particle deposition apparatus.
2. Related Art
In the same manner as optical fiber, the center portion of an optical fiber base material has a higher refractive index than the peripheral portion (cladding), and is referred to as the “core.” The primary structural component in optical fiber is SiO2, but the core is doped with GeO2 in order to increase the refractive index. In a normal manufacturing method for an optical fiber base material, a soot manufactured using a vapor synthesis technique such as vapor axial deposition (VAD), outer vapor deposition (OVD), or modified chemical vapor deposition (MCVD) is heated and changed into transparent glass.
With VAD, the glass raw material gas such as silicon tetrachloride is combined with glass fine particles as the result of a hydrolytic reaction with an oxyhydrogen flame, the glass fine particles are blown onto and deposited on a rotating starting member, and the starting member is pulled upward to be grown in the axial direction, thereby manufacturing the cylindrical soot.
FIG. 1 shows a state during manufacturing of the soot in which a plurality of burners are used to form the soot 2 by depositing the glass fine particles on the starting member 1. When a plurality of burners are used, a taper is formed on the soot 2 during manufacturing. The core deposition burner 3 forming the core is supplied simultaneously with silicon tetrachloride and a dopant raw material, such as germanium tetrachloride, and is therefore arranged separately from the first cladding deposition burner 4 and the second cladding deposition burner 5. A plurality of burners are often used to manufacture the soot 2. After the core positioned at the center is deposited on the starting member using the core deposition burner 3, the first cladding deposition burner 4 and the second cladding deposition burner 5 arranged thereabove are used to form the cladding that covers the outside. As a result, the portion where deposition is performed gradually gains a thicker diameter, and forms a tapered shape.
A variety of structures relating to the shapes of these burners have been proposed and are used. FIG. 2 shows the cross-sectional shape of a conventional burner 10. The burner 10 shown in FIG. 2 is an example in which a plurality of circular tubes with different diameters are arranged concentrically, thereby forming a plurality of ring-shaped gas flow paths. The glass fine particle gas and combustion aiding gas flow through the nozzle of the innermost circular tube 10a of the burner 10. Inert gas as silane gas, hydrogen gas, and combustion aiding gas flow sequentially, in the stated order, through the ring-shaped nozzles 11a to 11c formed by the adjacent circular tubes among the circular tubes 10b to 10d arranged outside the circular tube 10a, to be emitted from the tip.
FIG. 3 shows a cross-sectional state of another conventional burner 20. As shown in FIG. 3, in a rectangular multilayer tube formed by a plurality of rectangular tubes, beginning from the innermost tube, combustion aiding gas and raw material gas flow through the nozzle 21a, silane gas flows through the nozzle 21b, hydrogen gas flows through the nozzle 21c, and combustion aiding gas flows through the nozzle 21d, and these gasses are emitted from the tip of the burner.
FIG. 4 shows a cross-sectional state of another conventional burner 30. As shown in FIG. 4, a raw material gas emission flow path is arranged at the center of a circular multilayer tube formed by a plurality of cylinders, and a plurality of supplementary combustion aiding gas emission paths are arranged as a ring within a combustion aiding gas flow path formed to surround the raw material gas flow path, thereby forming a known multi-nozzle burner. The burner 30 includes a nozzle 31a that emits the glass raw material gas and the combustion aiding gas, a plurality of nozzles 31c that emit supplementary combustion aiding gas, and a ring-shaped nozzle 31d that emits hydrogen gas. In addition, the burner 30 includes nozzles 31b and 31e that emit inert gas as silane gas and a nozzle 31f that emits combustion aiding gas.
In recent years, due to advances in drawing techniques, it has become possible to quickly draw large optical fiber base materials. Furthermore, there has been increased worldwide demand for optical fiber, and there is a desire that large-scale optical fiber base material be manufactured in a short time. As shown in FIG. 1, when a plurality of burners are used to efficiently manufacture the soot 2, the burner flame for the cladding deposition contacts the tapered portion of the soot 2. As a result, the upper portion of the flame is emitted toward the thicker portion of the soot 2, and the lower portion of the flame is emitted toward the thinner portion of the soot 2. However, the burners shown in FIGS. 2 to 4 and described above each have a vertically symmetric structure with respect to the central axis of the burner. Therefore, the flame length and flame diameter at the top portion and bottom portion of the burner, the emission rate of the flame, and the like are the same, thereby decreasing the deposition efficiency and making it impossible to perform deposition with a suitable flame on the tapered portion of the soot 2. In order to improve the deposition efficiency, it is essential to realize a suitable flame strength division in combination with the tapered portion of the soot 2, such that the flame emitted toward the thicker portion of the soot 2 is stronger and the flame emitted toward the thinner portion of the soot 2 is weaker.
In light of the above problems, it is an object of the present invention to provide a glass fine particle synthesis burner, a method of manufacturing a glass fine particle deposition body, and a glass fine particle deposition apparatus, which can be used to efficiently deposit glass fine particles even on the tapered portion of soot.